Using ESTA.....for USA entry.?

I havn't visited the USA since the intoduction of ESTA in replacement of the old Visa Waiver system. I understand that you have to apply online for ESTA prior to your trip but my question is do you have to take a printed version of your authorisation to enter the USA with you to hand in at customs like the old Visa Waiver or do you just take your pport and no other doents. Thanks in advance...